Lazy load `tasks/toolchains/*.rake`

Improve performance `String#index` with UTF-8

Based on Boyer-Moore-Horspool algorithm (Quick Search algorithm).

As a side effect, the correct position is returned even if an invalid UTF-8
string is given.

```console
% ./mruby@master -e 'p ("\xd1" * 100 + "#").index("#")'
50
% ./mruby@improve-index -e 'p ("\xd1" * 100 + "#").index("#")'
100
```

The other behavior should be the same as the current implementation.

Remove an unused argument of `str_with_class()`

Set `MRB_STR_ASCII` flag when an inline symbol is stringized

Specialize Enumerable#max and Enumerable#min for Range

If no block is given and the Range has Fixnum or Float endpoints, do not iterate with
each and instead compare the endpoints directly. This implementation passes all of
the applicable specs from Ruby Spec.

Fix the order of "expected" and "actual" in `test/t/range.rb`

This patch prevents a hang for pathalogical (large) Ranges when
computing max and min.

Range inherits its implementation of max and min from Enumerable.
Enumerable implements max and min by calling each. For Range objects,
this is unnecessary since we know the max and the min by the end and
begin attributes.

It is also very slow. This code hangs unnecessarily:

    (0..2**32).max
    # ... hang
    (0..2**32).min
    # ... hang

This patch overrides max and min after including enumerable to yield
based on the begin and end methods.

Range#each depends on String#upto which is implemented in mruby-string-ext
which is why these tests live there.

Add macros for inline symbol for readability

Fix `Numeric#step` to infinity; ref. #4555

Fix missing `#ifndef MRB_ENABLE_ALL_SYMBOLS`

Merge pull request #4553 from shuujii/read-write-MRB_STR_ASCII-flag-only-when-MRB_UTF8_STRING-is-defined

Read/write `MRB_STR_ASCII` flag only when `MRB_UTF8_STRING` is defined

In #4550, @shuuji proposed the name name `MRB_STR_NO_MULTI_BYTE` for
more precise description. Although I agree that the name name is
correct, but the flag means the string does not contain multi byte UTF-8
characters, i.e. all characters fit in the range of ASCII.

Fix heap buffer overflow; ref #4549

This patch is showed in #4549.

Refine document to mrb_get_args()` [ci skip]

Fix and refine error message in `mrb_obj_to_sym()`

Before this patch:

  $ bin/mruby -e '1.respond_to?(2)'  #=> nil is not a symbol

After this patch (same as Ruby):

  $ bin/mruby -e '1.respond_to?(2)'  #=> 2 is not a symbol nor a string

Remove unused C header file from `src/etc.c`

Fix `include`, `prepend` and `extend` to frozen object